1. Field of the Invention
The present invention relates to a printing apparatus. More particularly, the present invention relates to electrophotographic printing apparatus, such as copiers and printers.
2. Description of the Related Art
In some electrophotographic printing apparatus, light-emitting devices, such as light emitting diodes and organic electroluminescence devices, are arranged linearly and are used as an exposure head. In such electrophotographic printing apparatus, a photoconductor surface is irradiated with light emitted by a light-emitting device array via a lens array, and a latent image is formed on the photoconductor surface by repeated exposure cycle in accordance with moving speed of the photoconductor. This exposure system differs from an exposure system in which a photoconductor is scanned with laser light using a polygon mirror. Use of the light-emitting device array helps reduce the size and noise of the printing apparatus.
The organic EL device has a characteristic that brightness thereof decreases after a long period of use. In a case in which brightness of the entire organic EL device array decreases uniformly, print quality is not seriously affected even after the brightness is decreased by about 10%. However, repeated printing with some organic EL devices emitting light for a longer time than others causes variation in degrees of brightness decrease in accordance with the position in the array. This causes stripe-patterned unevenness on print images and results in decrease in print quality even if brightness decrease is as small as 1% to 5%.
Japanese Patent Laid-Open No. 2006-346871 describes an invention related to printing apparatus including an LED array as an exposure head: in which printing apparatus, plural LED arrays are used sequentially one at a time when brightness of currently used one is decreased.
Life of each light-emitting device array is limited. Therefore, in such printing apparatus in which plural light-emitting device arrays are provided, increase in life of the entire light-emitting device arrays is achieved only by increasing the number of light-emitting device arrays. However, the number of light-emitting device arrays is not able to be increased greatly because the light-emitting device array forms an image on a photoconductor using a rod lens array and thus the number of light-emitting device is limited by the range of aperture of the rod lens array.
An electrophotographic printing apparatus having light-emitting devices as an exposure head in which a life of the exposure head is prolonged and thereby print quality is improved has been desired.